This invention relates to the field of borehole measurements. More particularly, this invention relates to new and improved apparatus for measuring weight, torque and side forces on a drill bit.
It is well known that the parameters of weight-on-bit (WOB), torque-on-bit (TOB) and side or bending force on bit (BOB) are important values to measure and/or control in the drilling of a well such as an oil or gas well. Measurement and control of WOB are important in regard to rate of penetration, rate of bit wear, and direction of drilling. WOB information can also be used to analyze drilling rate data to obtain information relating to formation porosity. Measurement and control of TOB are important in regard to drill bit wear; and measurement and control of BOB are important in regard to controlling the direction in which the borehole deviates from a straight path.
In the past it has been common to make WOB and TOB measurements at the surface. WOB has been measured at the surface by comparing off-bottom and drilling hookload weights. TOB has been measured by monitoring current supply to the rotary table drive motor or mechanical force on part of the rotary drive mechanism. However, such surface measurements are not highly reliable. Beyond that, it is not even possible to measure BOB at the surface. In the more recent past, particularly with the advent of reliable Measurement While Drilling (MWD) systems and telemetry systems for transmitting downhole measurements to the surface, proposals have been made to measure WOB and TOB downhole.
One of the more interesting of those recent proposals is disclosed in U.S. Pat. No. 4,359,898 in which strain gages are located in radial holes in a drill string segment. While the general concept of locating strain gages in radial holes is of interest and is used in the present invention, the system of that prior patent has complicated features that are eliminated in the present invention. For example, that prior patent requires a pressurized axial sleeve extending above and below the radial holes to eliminate stress concentrations in the drill string body in the vicinity of the openings resulting from pressure differentials of well fluids; and it also requires a sealed tube extending across the bore of the drill string which supports end plugs and, in conjunction with the end plugs establishes an atmospheric chamber in which the strain gages are located. Those features, while necessary for proper operation of the system of that prior patent, add significant complication and expense to the system.